Examining the Mechanical Quality of Stencils for Solder Paste Printing

In my work, I have summarized the process steps of reflow soldering technology with an emphasis on the stencil printing process. Build-up and raw materials of stencils, their characteristics and methods for tensioning were presented. The manufacturing procedures of stencils were thoroughly analyzed with taking their advantages and disadvantages into account. The qualification methods of stencils were also discussed where I presented a practical example of comparison and the laser triangulation method.

During my project work, I have made comparisons on two stencils -manufactured by DEK and Cookson-, which were produced with the same technology and are having identical nominal parameters. The printing quality was determined by measuring the area of the transferred solder paste printed onto the contact pads on the PWB. Aperture sets were examined: whose size is decreasing and another whose size is first increasing and then decreasing. Upon the results of this measurement, using the stencil manufactured by Cookson, the solder paste has covered a bigger area than using the DEK stencil. In some cases, the solder paste even did not get through the apertures (mainly the smaller ones) of the DEK stencil. Based on these experiments and the calculations made for defining paste transfer effectiveness, it turned out, that the Cookson stencil has higher quality, because using this one, it is possible to print with high quality even at small aperture sizes. Aperture sets for different sized BGA components were also investigated; where higher print area was achieved using the Cookson stencil.

I have executed slump tests as well, where the gap sizes are measured between two prints. During the measurements, the average of the gap sizes were higher at the DEK stencil, however, the deviation was higher than that of the Cookson stencil, so based upon the slump test, the two stencils were considered as having equal quality.

The stencil tensions were also examined, where the deflection of the stencils were measured. The tension of the Cookson stencil was little bit higher than that of DEK stencil, the difference was not significant, so they are considered as having the same tension values.

On the whole, the differences between the two stencils were only significant at the small aperture sizes, where using the Cookson stencil, I measured a higher area solder print on the contact pads of the PWB.